1. Technical Field
The present invention relates to a digital-analogue converter (referred to as a DA converter) that converts a digital signal into an analogue signal, and in particular relates to a current cell type DA converter.
2. Related Art
Generally semiconductor integrated circuit current cell type DA converters are controlled by a digital decoder input signal switching differential switches ON or OFF based on a decoded decode signal. The output current from the current cell adjusted by the differential switch is converted into a voltage level that accords with the current by an IV (current-voltage) conversion amplifier.
An example of such a DA converter is shown in FIG. 8. The conventional DA converter 100 shown in FIG. 8 is equipped with current a decoder 112, a current cell 120 including a current source 122, a current cell array 124, and a differential switch section 126, and a current-voltage converting section 128, including two IV conversion amplifiers A101 and A102.
An N-bit decoder input signal D100 controls PMOS transistors Pw0 to Pwn, Pz0 to Pzn (n=2N−1) of the differential switch section 126 by decoded decode signals S0Pw to SnPw and S0Pz to SnPz (n=2N−1), switching ON the P channel MOS (PMOS) transistors Pw0 to Pwn when the differential switches are in the ON state. A current path w1, which collects together the nodes that become the current path when the differential switches are in the ON state, is connected to one of the terminals of the IV converter amplifier A101, and is connected to the output side of the IV converter amplifier A101 via a feedback resistor R102.
In a similar manner, the PMOS transistors Pz0 to Pzn are switched ON when the differential switches are in the OFF state. A current path z1, which collects together the nodes that become the current path when the differential switches are in the OFF state, is connected to one of the terminals of the IV converter amplifier A102, and is connected to the output side of the IV converter amplifier A102 via a feedback resistor R103.
Current cell output currents Iw and Iz flow through the current paths w1 and z1, and are drawn in the respective amplifier stages 144 and 145 of the IV converter amplifiers A101, A102, via the respective feedback resistors R102 and R103 of the IV converter amplifiers A101, A102.
In the DA converter 100, there are large changes in the current amount drawn by the amplifier stages 144 and 145 of the IV converter amplifiers A101, A102 accompanying increases and decreases in the current cell output currents Iw and Iz due to changes in the decode signals S0Pw to SnPw and S0Pz to SnPz. Such changes are detrimental to the linearity of the output of the IV converter amplifiers A101, A102, and distortions occur in the output voltage waveform of the IV converter amplifiers A101, A102, with a worsening in the THD (Total Harmonic Distortion) characteristics.
A differential output type DA converter is known that is provided with a folding circuit between a current cell and an IV conversion amplifier in order to obtain good linearity (see for example Japanese Patent Application Laid-Open (JP-A) No. 2002-164788).
In the technology described in JP-A No. 2002-164788 the circuit structure is made complicated by provision of the folding circuit, the DA converter scale becomes large, and a problem arises of increased current consumption.